Nathan Freed Wessler sees it as a dangerous time machine as well as a communication device.

After all, says the ACLU attorney, the location data your phone gathers all day, every day, makes it possible for an interested party — the police, let’s say — to look back at a period of time and reconstruct precisely where you were. And from that to deduce who you were talking to and why.

In other words, imagine if the Department of Justice could have traced a Washington Post reporter to that Virginia garage where he met Deep Throat.

Such data may offer invaluable help in prosecuting a criminal. But if it’s available on demand, without a warrant, it could be a nightmare for journalists who are trying to protect their sources. And for their news organizations.

“It can really imperil the ability of journalists to do confidential reporting,” said Wessler, who will be in the Supreme Court on Wednesday to argue against the government in one of the most important privacy cases in many years: Carpenter v. United States.

In the case, FBI agents in Detroit got over four months’ worth of location records from cellphone-service companies for suspects in a robbery investigation. The ACLU is representing Timothy Carpenter, one of the suspects, who ended up being convicted.

No one is arguing that robbers shouldn’t be punished for their crimes. Rather, the argument is that, in an age when smartphones are ubiquitous, new privacy protections are necessary.

A long list of news organizations and journalism advocacy groups have signed friend-of-the-court briefs supporting the ACLU position: That law enforcement officials need a warrant before they are able to demand location data from cellphones.

“We’re not arguing that this information should be off-limits, rather making the modest claim that police should show probable cause and get a warrant first,” Wessler told me.“It’s a reasonable protection.”

Here’s why it matters to journalism: Reporters need confidentiality to do their work, and they need to be able to promise it to sources. At a time when President Trump has expressed the desire to crack down on sources and journalists, it’s not hard to imagine the implications.

A journalist’s cellphone location data can reveal “the stories a journalist is working on before they are published, where a journalist went to gather information for those stories, and the identity of a journalist’s source,” said the brief filed by the Reporters Committee for Freedom of the Press. Exposing those sources and journalistic methods “can put sources’ jobs and lives at risk, compromise the integrity of the newsgathering process, and have a chilling effect on reporting.”

Cellphones have become a kind of mobile newsroom, says Bruce Brown, executive director of the Reporters Committee, but there’s no way to use them without sharing location data with a service provider.

That paints a picture of where journalists go and with whom they meet. That “chills reporter-source relationships, threatens newsgathering, and ultimately harms the flow of information to the public,” Brown said.

In addition to the 19 journalism organizations supporting the ACLU’s position, the giants of the tech world are also on board: Apple, Verizon, Twitter, Facebook, Microsoft, Google and others.

Their brief rightly makes the case that “rigid analog-era rules should yield to consideration of reasonable expectations of privacy in the digital age.”

The criminal case relies on an outdated idea called the “third-party doctrine.” It says that by sharing information or records with a “third party” — such as a business or an individual — a person gives up any reasonable expectation that the information will stay private. That made sense in the 1970s when that action might have been canceled checks sent to a bank or a phone number dialed and transmitted over a phone company line.

But today, our phones send a stream of information at all times to a third party — the cellphone service provider.

Allowing that information to be accessed without a warrant might even be interpreted to mean that other digital information could be similarly unprotected, Wessler said — among the possibilities: “copies of notes, recordings of an interview,” or things that haven’t even been invented yet.

The Justice Department disagrees. It will argue that First Amendment protections for journalists will cover them adequately and that the third-party rule applies even in the digital age — that by the mere act of using their phones, people know they are giving up a certain amount of privacy.

Those who care about privacy and about protecting journalist-source relationships should hope that that argument fails.

If it doesn’t, law enforcement stands to gain what Wessler calls “an almost inconceivable new power.” A power that, in the digital age, would harm journalists and citizens.